security/gnupg: add the --shared-access option to scdaemon

gnupg's scdaemon opens smart cards in exclusive mode, which prevents other
applications (such as PKCS#11 libraries) from concurrently accessing the
card). Upstream refuses to fix the problem. This commit adds a
--shared-access option to scdaemon. When enabled, scdaemon will access the
smart card in shared mode, playing nicely with other applications. The
default behavior is unchanged.

See Also:
https://github.com/GPGTools/MacGPG2/commit/d6cb8039a0cdc74b9bdd89a3dfa93248aa2c4100
https://dev.gnupg.org/T3267
https://dev.gnupg.org/D320
https://github.com/OpenSC/OpenSC/issues/953

Reviewed by: adamw
Approved by: adamw (maintainer)
Obtained-from: GPGTools
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D22473